Legitimate ways to get Coins
EA permits Coins earned from eligible Ultimate Team matches and Coins received by selling items on the Transfer Market. FC 27’s launch update says eligible match Coin rewards and the share of tradable rewards were increased, but it does not give a fixed amount that every player receives.
Sources: EA Help: Coins and Points safety (opens in a new tab) · EA launch update (opens in a new tab)

What crosses the line
EA treats these as Coin distribution and lists possible enforcement ranging from Coin removal to restrictions on Ultimate Team or EA games.
- Do not buy Coins from a third-party seller.
- Do not send Coins to a friend through inflated or deflated item trades.
- Do not use giveaways or multiple accounts to distribute Coins.
Sources: EA Help: Coins and Points safety (opens in a new tab)
If you cannot use the market in an app
That can be an access-eligibility issue rather than a Coin shortage. The Web and Companion Apps have a separate Transfer Market access process; check the EA Account and active-day conditions.
Sources: EA Help: Transfer Market app access (opens in a new tab) · EA Help: Web and Companion App start (opens in a new tab)
Independent FC 27 guide. Linked official text supports the stated facts; any game imagery is promotional context.